The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Some communication systems transmit data from a transmitter to a receiver over multiple communication channels, using multiple transmit antennas and multiple receive antennas. Multiple channel transmission is used, for example, in spatial multiplexing schemes that achieve high throughput, in beam-forming schemes that achieve high antenna directivity, and in spatial diversity schemes that achieve high resilience against channel fading and multipath effects. These schemes are often referred to collectively as Multiple-Input Multiple-Output (MIMO) schemes.
In some MIMO schemes, a transmitter maps streams of modulated symbols onto spatial layers. The spatial layers are signals that are to be transmitted to a receiver over different MIMO transmission channels. The transmitter then applies a precoding operation to map each spatial layer onto a respective set of transmit antenna ports. The transmitter allocates a set of time-frequency blocks, referred to as resource blocks (RB), for transmission to a certain receiver. The signals within each resource block or multiple resource blocks are precoded using a respective precoding scheme, which specifies a mapping of spatial layers onto transmit antenna ports. Each resource block includes one or more dedicated (also called UE-specific) pilot signals, also known as dedicated reference signals (DRSs) which are precoded using the precoding scheme associated with the resource block. The resource block also includes one or more broadcast pilot signals, also known as cell-specific reference signals (CRSs), that are not precoded. In general, CRSs are transmitted in wideband (e.g., across the whole band which may be wider than the band allocated to a specific receiver) and intended for all the receivers within a cell. In general, DRSs are intended for a specific receiver and are transmitted only on the allocated resource blocks for the intended receiver.